1. Field of the Invention
The present invention relates in general to a pneumatic power brake system which permits the driver to easily brake the vehicle by exerting only a little pressure on the brake pedal, and more particularly to a pneumatic power brake system equipped with an electronic pressure control device by which the pressure applied to a work chamber of a brake booster is approximately controlled in accordance with a brake pedal force applied to the booster from the brake pedal.
2. Description of the Prior Art
Hitherto, various kinds of brake boosters have been proposed and put into practical use in the field of automotive brakes. Some of them are of a pneumatic type which uses a pressurized air as a medium for practically actuating a power piston of the brake booster. Japanese Patent First Provisional Publication No. 58-188746 proposes to incorporate the boosters of such type with an electronic pressure control means by which the pressure applied to the work chamber of the booster is electronically controlled in accordance with a brake pedal force applied to the booster from the brake pedal. In this connection, the Publication shows an input pressure detecting means mounted in the booster, which means detects the brake pedal force applied from the brake pedal. Since the output characteristic of the booster can be easily changed by only adjusting the control means, the brake systems of such type are widely applicable to various types of motor vehicles.
However, due to the inherent constructions of the control means, the conventional pneumatic power brake systems of such type hitherto proposed have failed to exhibit satisfactory performance. That is, they do not quickly feed stable pressure to the work chamber of the brake booster.